Contaminated groundwater has become an increasingly common problem facing industry, the government, and the general public. Leaks from below-ground storage tanks, above-ground storage facilities, surface discharges, and other sources contaminate groundwater and soil, posing a variety of problems to the general public as well as the environment. The most common types of contaminants are volatile organic compounds (VOCs) which are pollutants of petroleum based products and chlorinated solvents. Regulatory mandates have demanded assessment and remediation of the contamination; however, efficient and cost-effective solutions for containment and remediation are still needed.
Previously, contaminated groundwater was pumped to above-ground facilities, treated, and returned to the phreatic zone. These “pump-and-treat” or above-ground methods are expensive and generally treated the groundwater only. The pump-and-treat method does not provide treatment to the soil in the vadose zone.
Various in situ systems have been attempted, including air sparging, blast-enhanced fracturing, directional wells, in situ vacuum, groundwater recirculation wells, hydraulic and pneumatic fracturing, in situ flushing or injection/recirculation, in situ stabilization/solidification, permeable reactive barriers, and thermal enhancements. Various in situ biological treatments have included bioslurping, intrinsic bioremediation, monitoring natural attenuation, and phytoremediation. Additionally, various electrokinetic and electrolysis systems have been attempted. Each of these solutions has a variety of drawbacks, including the expense to implement and maintain the system, the level of effectiveness, and the potential for making the contamination worse.
For example, air sparging has been used to reduce concentrations of volatile organic compounds (VOCs) found in petroleum products. Air sparging is generally more applicable to the lighter gasoline constituents because they readily transfer from the dissolved to the gaseous phase. Appropriate use of air sparging may require that it be combined with other removal methods. Air sparging should not be used if the following site conditions exist: free product is present; near subsurface confined spaces; highly impermeable soils, nor when contaminated groundwater is located in a confined aquifer system.
If free product is present, due to the flow pattern created, air sparging can create groundwater mounding which could potentially cause free product to migrate radially away from the air sparge well; expanding the plume. When nearby basements, sewers, or other subsurface confined spaces are present at the site, air sparging should also not be used. Potentially dangerous constituent concentrations could accumulate in basements and other depressions unless a vapor extraction system is used to control vapor migration. When contaminated groundwater is located in the confined aquifer system, air sparging should also not be used. Air sparging cannot be used to treat groundwater in a confined aquifer because the injected air would be trapped by the saturated confine layer and could not escape to the unsaturated zone.
The effectiveness of air sparging depends primarily on two factors:
1. Vapor/dissolved phase partitioning of the constituents determines the equilibrium distribution of a constituent between the dissolved phase and the vapor phase. Vapor/dissolved phase partitioning is, therefore, a significant factor in determining the rate at which dissolved constituents can be transferred to the vapor phase.
2. Permeability of the soil determines the rate at which air can be injected into the saturated zone. It is the other significant factor in determining the mass transfer rate of the constituents from the dissolved phase to the vapor phase.
Stratified or highly variable heterogeneous soils typically create the greatest barriers to air sparging. Both the injected air and the stripped vapors will travel along the paths of least resistance (coarse-grained zones) and could travel a great lateral distance from the injection point. This phenomenon could result in the contaminant-laden sparge vapors migrating outside the vapor extraction control area.
U.S. Pat. No. 6,210,073 issued to Buehlman, et al. (Buehlman) attempts to provide a solution for highly stratified soil by implementing multi-level fluid transfer systems, or individual air-sparging supply lines for each stratification of soil. This process not only incurs multiple system costs, but also requires additional cost to accurately map the stratification. Error with respect to mapping the stratification results in an ineffective system, or worse, a remediation system that spreads the plume of contamination.
Prior art FIG. 1 illustrates an in-well vapor stripping process 100. This process creates a groundwater circulation pattern A. An air injection blower 110 injects air into an air injection line 120. The air exits the air injection line 120 below the water table percolating up through the well. A packer 130 forces the air-stripped water out through the upper recharge screen 140. The contaminated air 150 is collected through a vacuum extraction blower 160 and treated.
Limitations reported for this technology include limited effectiveness in shallow aquifers, possible clogging of the well due to precipitation and the potential to spread the contaminant plume if the system is not properly designed or constructed.
This technology pulls the groundwater up and beneficially increases the level of dissolved oxygen as it does so. However, the water is released at the top of the groundwater, and the bottom of the vadose zone; the region which already has the highest levels of dissolved oxygen.
The convection current generated acts to push the contaminate plume away from the well. Perhaps this final limitation is the most significant and fatal flaw. Spreading the contaminant plume in an already contaminated region is harmful to groundwater remediation, as it pushes contaminants even further from a contaminated region.
Accordingly, there is a need to address the limitations associated with air sparging and the other available in situ remediation techniques and to provide an effective, cost-efficient method and system for in situ remediation of contaminated groundwater and soil.